The figure represents three identical containers connected by valves that can be closed or opened to allow gas movement between the containers. At the beginning of a student's investigation, the valves are closed and the two outer containers are completely empty. The middle container holds particles of an ideal gas at a pressure of 9 atm. After the valves are opened and enough time has passed for net movement of particles between the containers to stop, the pressure of gas in the middle container will be closest to which of the following?
1.5 atm
2 atm
3 atm
9 atm